There are certain situations in telecommunications where a phase reversal is used to signal a particular condition. For example, in ITU (International Telecommunications Union) standard G.165 entitled "General Characteristics of International Telephone Connections and International Telephone Circuits", a 2100 Hz tone signal that has a phase reversal every 450.+-.25 ms is used to disable an echo canceller. A valid phase reversal is defined as a phase variation in the range of 180.degree..+-.25.degree. of a 2100 Hz (.+-.21 Hz) tone. An invalid phase reversal is defined as a phase variation in the range of 0.degree..+-.110.degree..
Some means must be provided for detecting valid phase reversals and rejecting invalid phase reversals. The detector employed must operate perfectly on signals having a level of -31 dBm.o slashed. to 06 dBm.o slashed. in conditions of white noise less than or equal to 11 dB below the level of the 2100 Hz tone signal. For white noise levels between 11 dB and 5 dB below the level of the tone signal, the percentage of correct operation should fall by no more than 1% for each dB.
The tone disabler is required to operate (disable the echo canceller) within one second of the receipt of the disabling signal.
While the G.165 standard specifies additional requirements, reliable phase reversal detection is essential for the proper functioning of the echo canceller tone disabling detector.